Radiation gauge techniques have been used in the prior art to measure weight percentages, or parameters related to weight percentages (e.g., fiber volume), for the constituents of certain kinds of composite materials such as fiberglass-epoxy composites. Technical Report AFML-TR-68-233 entitled "Development of Nondestructive Tests for Predicting Elastic Properties and Component Volume Fractions in Reinforced Plastic Composite Materials", published by the Air Force Materials Laboratory, Air Force Systems Command, Wright-Patterson Air Force Base, Ohio in February 1969, describes radiation gauge techniques that have been used for non-destructively determining weight percentages of the constituents of composite materials.
In general, accurate measurements for weight percentages of the constituents of a composite material have been obtainable in the prior art only in cases where the constituents of the composite material have different atomic numbers, and where the radiation absorption coefficients of the constituents are significantly different from each other for at least one wavelength. For composite materials such as graphite-epoxy composites, the radiation gauge techniques developed in the prior art have generally been unable to provide accurate measurements for constituent weight percentages and for fiber volume.
A graphite-epoxy composite essentially comprises graphite fibers (pure carbon) and an epoxy resin, which are mixed in specified proportions according to the particular application. Epoxy resin is typically composed of carbon (approximately 70 percent by weight) and varying quantities of other elements such as hydrogen, nitrogen and oxygen. From the standpoint of distinguishing quantitatively between the carbon fibers and the epoxy resin in a graphite-epoxy composite, the epoxy resin can be characterized as an impure carbon, i.e., carbon with an admixture (typically 30 percent by weight) of other elements. Unfortunately for radiation gauge measurement techniques of the prior art, the presence of the other (i.e., non-carbon) elements in the epoxy resin typically does not sufficiently change the value of the effective radiation absorption coefficient of the epoxy resin (i.e., impure carbon) from the value of the radiation absorption coefficient of the carbon fibers (i.e., pure carbon).
Graphite-epoxy composites, which are currently attracting attention for use in, e.g., aircraft and space vehicle structures, provide examples of particular kinds of composite materials for which radiation gauge techniques of the prior art were ineffective in measuring constituent weight percentages and related parameters. If the epoxy resin in a graphite-epoxy composite is considered as a single constituent (viz., impure carbon), the effective radiation absorption coefficient of the epoxy resin is not sufficiently different from the radiation absorption coefficient of pure carbon to enable radiation gauge techniques known in the prior art to distinguish unambiguously between the epoxy resin and the pure carbon.
In practice, the measurement of graphite fiber content by weight percentage or by volume in graphite-epoxy composites has heretofore been accomplished by destructive testing methods. A preferred destructure testing method for making such measurements was described in Technical Publication ASTM D3171 published by the American Society for Testing and Materials in the "Annual Book of ASTM Standards", Part 36, (1984).